1. Field of the Invention
The invention relates to an excitation wavelength sweeping type Raman spectroscopic apparatus that measures a Raman spectrum of a specimen by sweeping a wavelength of a laser beam.
2. Description of the Related Art
The Raman spectrum, together with an infrared absorption spectrum, is known as a method of measuring a molecular vibration spectrum of gases, liquids, and solids. The Raman spectrum is widely used as a method of evaluating materials.
Raman scattering is a phenomenon in which when monochromatic light (excited light) with a frequency .upsilon..sub.0 is irradiated onto a material, light of different frequencies (.upsilon..sub.0 .+-..upsilon..sub.1, .upsilon..sub.0 .+-..upsilon..sub.2) is scattered as shown in FIG. 2, other than intense scattered light (Rayleigh scattering) whose frequency is the same as that of the irradiated light. Since a difference (.upsilon..sub.1, .upsilon..sub.2 . . . ) between the frequency of the irradiated light and that of the scattered light corresponds principally to the energy difference between the initial state and the final state, Raman spectroscopic apparatuses employed up to now are designed to obtain a Raman spectrum by fixing .upsilon..sub.0 and dispersing the scattered light with a double monochromator or a triple monochromator.
In these conventional Raman spectroscopic apparatuses, the light obtained by converging the Raman scattered light passes through the entering slit and the exiting slit of the spectroscope to reach the detector, reducing the amount of light to such an extent that the light becomes dark and the transmittance gets impaired. As a result, these conventional spectroscopic apparatuses are not well qualified to detect Raman scattered beams efficiently, nor are they advantageous in that they are heavy and so large that a sufficient space must be provided. In addition these spectroscopic apparatuses are expensive.
Therefore, there has been a demand for developing a highly sensitive Raman spectroscopic apparatus whose efficiency for detecting Raman scattered light is high to replace conventional Raman spectroscopic apparatuses. The invention has been made in view of such a demand.